Page:The New International Encyclopædia 1st ed. v. 01.djvu/637

ANEMOMETER. made by Hagemann, of Denmark, and by Dines, of England, appear to be especially appropriate to the measurements of gusts. The combination of suction-anemometer, pressure-anemometer, and aneroid barometer recommended by Professor Cleveland Abbe in 1882, and especially the application to the tube of parallel plates that entirely annul the wind effects seem to be essential if we would determine the true barometric pressure with a barometer exposed to the wind, as, for instance, on a mountain top.

Rotation-anemometers are those in which the wind sets in motion plane or curved metallic blades. The earliest form resembled that of Dinglinger, mentioned by Leupold in 1724, in that it used the Polish water-wheel with vertical axis, but differed essentially in that Dinglinger prevented the rotation of the arms and measured the pressure required to keep them quiet, whereas d'Ons-en-Bray, in 1734, allowed them to rotate continuously. Since that time two essentially different varieties of the rotation-anemometer have been developed, namely (a) those of Schober and Woltmann, Combes, Casella, Whewell, or Biram, in all which sets of plane plates inclined to an axis are forced to revolve about it by the wind blowing in the direction of the axis. This form is much used in studies on ventilation of mines and buildings. The most important meteorological application of this style is that manufactured by Richard for use at the French observing stations. (b) The Robinson anemometer, brought out by Dr. Robinson in 1846, but suggested to him by Edgeworth many years before. This has come into very general use by English and American meteorological observers as the Robinson hemispherical cup anemometer. In this instrument a vertical spindle carries at its upper end four horizontal arms at right angles to each other; each arm carries at its extremity a hollow hemispherical cup of thin sheet metal whose circular rim is in a vertical plane passing through the common vertical axis of rotation of the spindle. The wind rotates these cups so that the convex side of each cup goes forward. Numerous experiments have been made to determine the relation between the velocity of the wind and that of the cups. The instrument makers have generally followed Dr. Robinson's conclusion, that the linear motion of the centre of the cup is one-third of that of the wind; but observation and experiment, as well as theory, show that this cannot be true. The most intelligent and satisfactory investigation of this important subject has been carried out by Professor C. F. Marvin, of the United States Weather Bureau. Combining his results with those of European students, we must conclude that in perfectly uniform winds the general average ratio between the velocity of the wind and that of the cups varies with the length of the arm and the size of the cups between 2.5 and 3.5, so that it is necessary to determine the ratio by actual experiment upon each respective type of anemometer.

ROBINSON ANEMOMETER.

Professor Marvin shows, besides, that the ratio varies according as the anemometer is exposed to a uniform wind or to one that is variable and gusty. He finds that in the latter case the ratio depends not merely upon the dimensions of the arms and cups, but especially upon the moment of inertia of the revolving system; that is to say. on the mass of the cups. For gusty winds, the recorded wind velocity is always too great. This is explained by the fact that the gusts give to the revolving cups a great velocity, which they, by reason of their momentum, retain after the gust has ceased. It would seem, therefore, that rotating anemometers should be standardized not merely in quiet air, but also out of doors in ordinary gusty winds. By such comparisons Professor Marvin has compiled a table, of which the following is an abstract, showing the correct wind velocity for records of anemometers in the ordinary or average gustiness of the wind at Washington. If the observed wind velocities are indicated on dials constructed on the assumption that the centres of the cups move with one-third the velocity of the wind, then the corrected wind velocities are given by the following table:

Weather Bureau Marvin's Equiva- Correspond ing Anemometer. In- lent. Correct Ve- Pressure in pounds dicated ^ elocity. locity. Miles per on one square foot Miles per hour. hour. of area. .1 .1 .3.8 .8 .8 .9 .6 .6 .1 .5 .4 .9 .6 .6 .7 .6 .8 .2

Observations on strong winds on the summit of Mount Washington indicate that the velocities given in this table apply also to that high elevation, so that there is no evidence that the Robinson anemometer is appreciably influenced by changes in the density of the air; but, of course, the wind pressures for a given velocity are smaller in proportion to the density. In order to determine the coefficient for computing wind pressure at high velocities, Marvin conducted special measurements at the summit of Mount Washington, using both large and small-pressure plates, and obtaining automatic simultaneous records on the same sheet of paper for both the pressure and the velocity. He finds that when the air has the standard density for 32° F. and 30 inches of pressure, the wind pressure on a plane flat surface is equal to 0.0040 pound to the square foot multiplied by the square of the velocity of the wind in miles per hour and by the area of the plate; this formula gives the pressures printed in the preceding table. (For further details, see Professor Marvin's paper on wind-pressures and wind-velocities, printed in the annual report of the chief signal officer of the army for 1890.) A general review of the subject of anemometry is given in Abbe's Treatise on Meteorological Apparatus and Methods (Washington. 1887). The Robinson anemometer, as originally manufactured by James Green, of New York, and reduced